The invention relates to an aqueous liquid solution of a betaine of the general formula ##STR2## in which R is an alkyl group of coconut fatty acids, preferably hydrogenated, or a fatty acid mixture which, on the average, corresponds to coconut fatty acids, the solution having a solids content of at least 40% by weight, a pH of 5 to 8 and an aminoamide content of not more than 1% by weight.
More particularly, the invention relates to aqueous liquid solutions of a betaine of the aforementioned type with a solids content of at least 42% by weight and particularly of at least 45% by weight. The solids content is defined as the weight which is determined by evaporating sample on a flat glass disk for 2 hours at 105.degree. C.
It is well known that solutions of betaines of the aforementioned type are liquid only below a particular concentration of total solids, which depends on the fatty acids or fatty acid mixtures used for their preparation. For example, a solution of a betaine derived from coconut fatty acids solidifies at a solids content of about 40% by weight. For this reason, conventional, commercial, aqueous solutions of coconut amidopropylbetaine, derived from coconut fat, have total solids concentrations which are clearly below 40% by weight and in most cases about 35% by weight. The maximum achievable concentration of a still flowable solution of a betaine decreases as the number of carbon atoms is increased. If the fatty acid mixture contains a higher proportion of unsaturated fatty acids, the concentrations achievable frequently are comparatively higher than those achievable with saturated fatty acids.
A conventional commercial betaine solution typically has the following composition:
______________________________________ water: 64% by weight betaine: 30% by weight NaCl: 5% by weight glycerin: 0.3% by weight ca. 36% by weight fatty acid: 0.5% by weight total solids aminoamide: ca.0.3% by weight ______________________________________
In this sense, the solids content represents the sum of the components other than water. The proportions of betaine and sodium chloride arise out of the stoichiometry of the reaction of the fatty amide with tertiary amino groups (aminoamide) and sodium chloroacetate according to: ##STR3## A small amount of aminoamide normally remains in the product because the reaction is incomplete. This proportion can, however, be reduced further by an adapted stoichiometry and reaction procedure. The further typical components listed originate from the synthesis of the aminoamide. If the aminoamide has been obtained by the reaction of the fatty acids with 3-N,N-dimethylaminopropylamine, residual amounts of fatty acids are still present. If the aminoamide has been synthesized from fats and 3-N,N-dimethylaminopropylamine, portions of fatty acids as well as glycerin are present.
The aminoamide, if anything, is unwanted for physiological reasons and therefore every effort is made to keep the content thereof low. On the other hand, glycerin and fatty acids are desirably used and frequently in cosmetic preparations. In the case of glycerin, primarily skin care products prepared therewith are esteemed. Fatty acids have good effects on formulations. For example, A. L. Hunting describes the use of fatty acids as thickening agents in shampoo formulations in "Cosmetics and Toiletries" 97, 53 (1982). Particularly in the form of their salts, an increase in viscosity is brought about by their use.
There has been no lack of attempts to produce betaine solutions of higher concentration. However, such attempts have been successful only if extraneous compounds are used or special methods are employed, which increase the cost of the product.
For example, in U.S. Pat. No. 4,243,549, pourable aqueous preparations are described which contain, for example, 33.5% by weight of a betaine, 33.5% by weight of an ethoxylated sodium alkyl sulfate, 9% by weight of sodium chloride and 2 to 3% by weight of impurities, the remainder being water. These mixtures exist in the form of the so-called G phase. The flowability of such mixtures is based on the special micellar structure of the G phase, which exists only within a relatively narrow concentration range. However, the required presence of equal amounts by weight of anionic surfactants is undesirable for many formulations in which betaine solutions are used. Even if anionic surfactants are added for a later application, the user himself wants to determine which anionic surfactants are used. Mixtures with cationic surfactants are not possible, because such surfactants interact with anionic surfactants.
German patent 36 13 944 relates to a method for the preparation of flowable and pumpable solutions containing at least 70% by weight of betaine. For the synthesis of the betaine,
a) the ammonium salt of the halogenated carboxylated acid is used as salt, PA0 b) the quaternization is carried out in a polar organic solvent, which may not contain more than 20% by weight of water, PA0 c) after the quaternization, any water contained is distilled off azeotropically and the precipitated ammonium halide is removed, after which PA0 d) the solvent is distilled off partly or completely and PA0 e) before, at the same time as or after the distillation, the concentration of the betaine in the solvent or solvent mixture desired for the application is adjusted to the desired value.
The need to use solvents and to remove the precipitated ammonium chloride, for example, by filtration, has proven to be a disadvantage of this method. Moreover, the addition of preferably aliphatic diols in amounts of 2 to 15% by weight based on the solution, and of 0.5 to 10% by weight of ethanol was recommended in order to attain the desired, low viscosity. The diol and/or ethanol content is also not always desirable and tolerable.
The object of German patent 37 26 322 is a method for the synthesis of the above-named betaines in the form of concentrated, aqueous solutions. The concentration of the preferably still hot solution obtained after the quaternization is adjusted to the desired value, if necessary, by evaporation of water, and mineral acid is added to the solution and, before or after the concentration is adjusted to the desired value, mineral acid is added to the solution in such amounts that the pH of the solution is 1 to 4.5. However, corrosion problems, which make the use of acid resistant material necessary for storage tanks, tank cars, etc., arise during the storage, transport and use of the acidic betaine solutions.
Finally, reference is made to the German patent 38 26 654, according to which concentrated betaine solutions are obtained owing to the fact that: nonionic, water soluble surfactants are added to the reaction mixture before or after the quaternization reaction or to the solution of betaine obtained in such amounts; the finished solution contains 3 to 20% by weight of nonionic surfactants and an adjustment of the pH of the solution by the addition of alkaline solution to a pH of .gtoreq.5 to 9 is precluded after the quaternization. In this case also, the betaine solution contains an extraneous, nonionic surfactant, the presence of which can be undesirable during the later use of the betaine solution.